MAX1703 Maxim, MAX1703 Datasheet - Page 10

• Manufacturer Part Number: MAX1703
• Description: 1-Cell to 3-Cell / High-Power 1.5A / Low-Noise / Step-Up DC-DC Converter
• Manufacturer: Maxim

The MAX1703 can be synchronized in PWM mode to a
200kHz to 400kHz frequency by applying an external
clock to CLK/SEL. This allows the user to set the har-
monics to avoid IF bands in wireless applications. The
synchronous rectifier is also active during synchronized
PWM operation.
Pulling CLK/SEL low places the MAX1703 in a low-
power mode. During low-power mode, PFM operation
regulates the output voltage by transferring a fixed
amount of energy during each cycle, and then modulat-
ing the pulse frequency to control the power delivered
to the output. The devices switch only as needed to
service the load, resulting in the highest possible effi-
ciency at light loads. Output current capability in PFM
mode is 150mA (max). The output voltage is typically
1% higher than in PWM mode.
During PFM operation, the error comparator detects the
output voltage falling out of regulation and sets a flip-
flop, which turns on the N-channel MOSFET switch
(Figure 4). When the inductor current ramps to the PFM
mode current limit (800mA typical) and stores a fixed
amount of energy, the current-sense comparator resets
a flip-flop. The flip-flop turns off the N-channel switch
and turns on the P-channel synchronous rectifier. A
second flip-flop, previously reset by the switch’s “on” sig-
1-Cell to 3-Cell, High-Power (1.5A),
Low-Noise, Step-Up DC-DC Converter
Figure 3. Simplified PWM Controller Block Diagram
10
REF
CURRENT
2.7A TYP
FB
LIMIT
OSC
______________________________________________________________________________________
Synchronized PWM Operation
Low-Power PFM Operation
S
R
Q
POUT
P
N
PGND
LXP
LXN
nal, inhibits the error comparator from initiating another
cycle until the energy stored in the inductor is transferred
to the output filter capacitor and the synchronous rectifier
current has ramped down to 80mA. This forces operation
with a discontinuous inductor current.
The MAX1703 features an internal 140mΩ, P-channel
synchronous rectifier to enhance efficiency. Synchro-
nous rectification provides a 5% efficiency improve-
ment over similar nonsynchronous boost regulators. In
PWM mode, the synchronous rectifier is turned on dur-
ing the second half of each switching cycle. In low-
power mode, an internal comparator turns on the
synchronous rectifier when the voltage at LX exceeds
the boost regulator output, and then turns it off when
the inductor current drops below 80mA.
The MAX1703 uses a CMOS, low-voltage start-up oscil-
lator for a 1.1V guaranteed minimum start-up input volt-
age at +25°C. On start-up, the low-voltage oscillator
switches the N-channel MOSFET until the output volt-
age reaches 2.15V. Above this level, the normal boost-
converter feedback and control circuitry take over.
Once the device is in regulation, it can operate down to
a 0.7V input, since internal power for the IC is boot-
strapped from the output via the OUT pin. Do not apply
full load until the output exceeds 2.3V (max).
Figure 4. Controller Block Diagram in Low-Power PFM Mode
800mA TYP
REF
CURRENT
FB
COMPARATOR
LIMIT
ERROR
Low-Voltage Start-Up Oscillator
S
Q
R
R
S
D
Q
Synchronous Rectifier
LOGIC HIGH
P
POUT
N
PGND
LXP
LXN